Trachymyrmex smithiHNS might be confused with T. jamaicensisHNS due to its large size and dark coloration, but the species are allopatric; T. jamaicensisHNS is only known from southwest Florida, the Florida Keys, and the Caribbean, whereas T. smithiHNS occurs in the deserts of western Texas, New Mexico, and the State of Coahuila in northern Mexico (see distribution maps). In addition, the frontal and preocular carina of T. smithiHNS do not form a well developed antennal scrobe that extends back to the preoccipital margin as in T. jamaicensisHNS , and the frontal lobes are triangular in T. smithiHNS , not rounded, as in T. jamacensisHNS .

Buren (1944) described T. smithiHNS from Mexico (La Rosa, Coahuila) and Cole (1952) later based the subspecies T. smithi neomexicanusHNS on workers from the United States (Las Cruces, New Mexico). Cole separated neomexicanusHNS from smithiHNS because it possessed larger and less tuberculate spines, a more concave posterior margin of the postpetiole, larger body size, darker color, and more abundant gray "granulation" on the integument. For a TrachymyrmexHNS , T. smithiHNS workers show considerable size variation and all characters, except color and the presence of granulation, vary proportionally to size. The morphological differences cited by Cole for neomexicanusHNS fall well within the range of variation shown by this widely distributed species. Likewise, black, reddish-brown and intermediate color morphs are distributed over the species entire range, including the type locality (C. Rabeling personal observation). Molecular evidence also supports our contention that in T. smithiHNS we are dealing with a single variable species. The short branch lengths in the molecular phylogenetic analysis (see below) show that the sequence diversity is minimal and similar for both smithiHNS and neomexicanusHNS (Figure 21). The more abundant gray granulation of neomexicanusHNS mentioned by Cole (1952) is most likely caused by actinomycete bacteria of the genus Pseudonocardia (Cafaro & Currie 2005), which grow on the ant 's exoskeleton. Actinomycete load on the worker 's body surface varies among individuals of the same nest and is affected by worker age, characteristic activity (foraging versus garden-tending), or the health of the fungus garden. This coating is therefore of no taxonomic importance. To remove the sometimes confusing actinomycete coating, specimens can be washed with acidic acid (vinegar).

To our great surprise we encountered two holotypes of T. smithi BurenHNS ; one deposited in the LACM and the other in the USNM collection. Most likely, the USNM specimen was mislabeled and actually represents a paratype, because Buren (1944, p. 6) stated that the single holotype would remain in his personal collection and paratypes would be deposited in the National Museum and his personal collection. Since the LACM accessioned the W. F. Buren collection in 1983, we here designate the holotype of T. smithiHNS as the specimen deposited at the LACM.

Etymology

Buren (1944) named this species after Marion R. Smith, myrmecologist and curator of Hymenoptera at the National Museum of Natural History in Washington, DC for many years during the mid twentieth century.

Biology

Trachymyrmex smithiHNS is a Chihuahuan Desert species that occurs in southwest Texas, south-central New Mexico, and the Mexican states of Chihuahua and Coahuila.

It inhabits creosote bush bajadas (alluvial fans), mesquite/ YuccaHNS grassland playas (dry lakebeds) and mesquite coppice dune habitats at elevations of 1100- 1500 m. Nests are often in the shade of creosote bush or Mormon tea ( Ephedra trifurca ). Older nests may have large nest mounds (~30 cm diameter) with conspicuous middens consisting of dried leaves and exhausted fungus substrate. The subterranean nests of T. smithiHNS are the largest of all TrachymyrmexHNS species occurring in the US. Older nests consist of more than 20-30 chambers (Johnson et al. 2006; Rabeling & Mueller, unpublished data) of which 50-60% contain fungus gardens in the summer. Near El Paso, Texas, the fungus gardens are nourished with entire mesquite leaflets (Johnson et al. 2006; Rabeling, unpublished data), and resemble the fragile fungus gardens of grass-cutting AcromyrmexHNS species in South America. Colonies can be very populous; Johnson et al. (2006) report up to 786 workers and 6 dealate queens in one nest, and Schuhmacher and Whitford (1974) estimate 1250 workers for one colony based on mark-recapture experiments. Colony activity and the number of fungus gardens decrease from November through May (Schuhmacher & Whitford 1976). Two nests, which we partially excavated during winter, had 26 chambers per colony, reaching down to 180 and 130 cm depth, respectively. None of the chambers contained a fungus garden in December, suggesting that T. smithiHNS moves its gardens to deeper layers in winter. During springtime the excavation of two adjacent colonies showed, that in April the ants already moved the fungus garden to shallower nest chambers. Numerous fungus gardens were encountered hanging from the chambers ' ceilings in 25-130 cm depth. From the partially excavated winter colonies, 212 and 362 workers were collected (Rabeling & Mueller, unpublished data). Trachymyrmex smithiHNS forages mostly at night during the summer months, to avoid soil temperatures exceeding 50°C during the day (Whitford 1978).